1. Field of the Invention
The present invention relates to a piezoelectric vibrator which is used in, for instance, frequency voltage-controlled oscillators (VCOs) and more particularly to an AT-cut quartz resonators in which the capacitance ratio .gamma. is reduced by the use of an inverted-mesa electrode.
2. Description of Prior Art
Generally, quartz crystal resonators have a high Q value, so that the frequency stability of such resonator is high. The resonators are widely used in radio apparatus as reference oscillators, etc. AT-cut quartz resonators are particularly superior in terms of temperature-frequency characteristics.
As shown in FIG. 22, in a conventional an AT-cut quartz resonator, disk-form electrodes 3a and 3b, which are smaller than the diameter of a circular AT-cut quartz crystal substrate 1, are provided on both sides of the central portion 2 of the substrate 1, and lead wires for supplying voltage are led out from these electrodes 3a and 3b. Here, a parallel-plane substrate or a substrate which has been subjected to bevel working, etc. is used for the substrate 1; and gold, silver, aluminum, nickel or alloys of these metals, etc. are used as the electrodes 3a and 3b.
The AT-cut quartz resonator described above is generally used in a frequency voltage-controlled oscillator (VCO) in which the oscillation frequency is controlled by a voltage from the outside, thus constituting a frequency voltage-controlled quartz crystal oscillator (VCXO). The AT-cut quartz resonators used in such applications should therefore offer not only a broad adjustable range of frequencies but also an excellent frequency stability when the control voltage is constant.
However, the VCXO that uses quartz resonators has problems: although the frequency stability is high due to the fact that the Q value is higher than that of other piezoelectric vibrators, the frequency adjustable range is narrow, thus making it difficult to satisfy the above-described requirements.
The AT-cut quartz resonators, that is, vibrators formed by providing electrodes of a uniform thickness to an AT-cut quartz substrate which has a parallel plane shape or which has been subjected to bevel working (as shown in FIG. 22) are used in such VCXOs, the frequency adjustable range is narrow since the characteristics capacitance ratio .gamma. of the AT-cut quartz resonator has a minimum limit value of approximately 200 and cannot be lowered any further than such a value.
When the capacitance ratio .gamma. of such an AT-cut quartz resonator is reduced, there are such merits as the short-term stability is improved, the frequency adjustable range is broadened when a filter construction is employed, impedance can be lowered and excitation is facilitated in addition that the frequency adjustable ranges of the VCO is broadened. Nevertheless, it has been considered impossible in the past to lower the capacitance ratio .gamma. of the AT-cut quartz crystal as oscillator described above to a value of 200 or lower.
The present invention is devised in light of the above facts; and the object of the present invention is to provide an AT-cut quartz resonator which makes it possible to reduce the capacitance ratio .gamma. to a value smaller than the conventional characteristic capacitance ratio .gamma., that has been recognized by a person skilled in the art, by using inverted-mesa electrodes.